The present invention relates to a process for the thermal treatment of a polysilazane and, more particularly, the treatment of a polysilazane containing .tbd.SiH groups and ##STR2## groups.
Organopolysilazanes, referred to as polysilazanes hereinafter, are well-known products that occur in the form of monomers, oligomers, cyclic or linear polymers, and resinous polymers. These polysilazanes can be prepared using a wide variety of methods from a wide range of starting materials.
Particularly, these polysilazanes can be shaped and pyrolyzed in the form of Si.sub.3 N.sub.4, SiC or their mixtures. The silicon nitride can also be manufactured by reacting chlorosilanes, such as SiCl.sub.4, HSiCl.sub.3, and H.sub.2 SiCl.sub.2, with ammonia at a high temperature and in a vapor phase. It is difficult to manufacture shaped articles, especially fibers, because this method directly yields a powdered product. In contrast, polysilazanes can also be spun into continuous fibers that yield ceramic fibers upon pyrolysis.
The polysilazanes can be shaped into films of various gauges and massive molded articles. They can also be used as binders for ceramic fiber or carbon fiber and as sintering binders for porous ceramic articles.
However, difficulties are encountered in converting these polysilazanes readily and economically into the form of fibers or coatings that, after pyrolysis, yield ceramic products in the form of fibers, films, coatings of various gauges and molded articles.
Japanese patent application No. 77/160,446 describes a process for the polymerization of an organopolysilazane of high molecular weight by using acidic earths as a catalyst for the treatment of the organopolysilazane. However, in this process, the solid catalyst needs to be separated off by filtration. This separation involves the use of a solvent in the case of polymers that can reach high viscosities.
One attempt to solve this problem was made in U.S. Pat. No. 3,853,567. This patent discloses a process that manufactures shaped articles, such as fibers, comprising silicon carbide, silicon nitride or mixtures thereof, and other ceramic products. The process carries out the first thermal treatment of a polysilazane at a temperature between 200.degree. C. and 800.degree. C. to obtain a meltable carbosilazane resin that can be melt-spun and then pyrolyzed at a temperature between 800.degree. C. and 2000.degree. C.
Although the process of this patent represents a significant step forward, it requires a first thermal treatment at a temperature that is very high, such as 200.degree.-800.degree. C. The weight yield of ceramic from this process is typically inadequate.
U.S. Pat. No. 3,892,583 describes a process for the preparation of shaped articles comprising a homogeneous mixture of silicon carbide and nitride. In this process, a polysilazane is prepared by reacting ammonia with a halosilane at a temperature ranging up to 200.degree. C. to form the silazane. The article is then treated at a temperature between 200.degree. C. and 800.degree. C. This patent indicates that the thermal treatment makes it possible to obtain a product that is more highly crosslinked and has improved strength. Column 3, lines 13 to 19; and column 5, lines 5 to 13. However, there is no mention anywhere in this patent that, in order that the thermal treatment may be effective at a low temperature, that is to say at a temperature below 220.degree. C., the polysilazane needs to contain on average at least two, preferably at least three, .tbd.SiH groups and at least two ##STR3## groups per molecule.